I've tried many times with a 400mm focal length and I get a very crude image at best. My guess, based on what I know about telescopes, is you'll need at least 1500mm effective focal length to get reasonable detail, or you could try some filters to keep the moon from burning out the center of the image. Another suggestion is to hook up to a digiscope, or a high-quality binocular using an attachment provided by the mfr.

_________________Jake O'Connell, 40D CrewCanon EOS 40D | 28-135mm IS | 50mm f/1.8 |Vivitar DF 383 | Vivitar 285hvMy Flickrmy Blog"Photography isn't so much about the results as it is the collective experience, your interactions with people and with the world"